The World Health Organization (WHO) has estimated that approximately 5–10% of all respiratory infections worldwide are caused by influenza. Annually, 3–5 million people also develop a severe illness caused by influenza.
Influenza is transmitted primarily through person-to-person contact with droplets traveling up to 6 feet away. Transmission can also occur through contact with contaminated surfaces and touching a mucosal surface such as the nose, mouth, or eyes. It is thought that people may be able to transmit the virus up to a day or two before showing symptoms and up to 5–7 days after the start of the illness. Influenza-related symptoms may include fever and/or chills, runny nose or congestion, head and body aches, and general fatigue.
The influenza virus is an enveloped, negative-sense RNA virus in the Orthomyxovirus family. It is generally 80–120 nanometers (nm) in diameter. The internal structure proteins determine the three human subtypes (A, B, C). Types A and B are seasonal or pandemic, whereas type C tends to be a mild respiratory illness.
Type A is divided into subtypes and strains based on the two surface "spikes" of glycoproteins. H or HA (hemagglutinin) has 18 known subtypes, and N or NA (neuraminidase) has 11 known subtypes. This combination creates the strains (i.e., H1N1, also known as swine flu, and H5N1, the avian flu). In addition to the glycoproteins, there are also M2 ionic channels along with matrix protein (M1) located just under the outer lipid layer. This M1 protein makes up 40% of the viral protein.
Figure 1. PHIL Public Health Domain/Higgins, D. (2014). This is a 3-dimensional illustration highlighting the different features exhibited by an influenza virus, including the surface proteins, hemagglutinin (HA) and neuraminidase (NA). Centers for Disease Control and Prevention. https://phil.cdc.gov/Details.aspx?pid=17346
